<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>data 选项</title>
    <script src="js/vue.global.js"></script>
</head>
<body>
<div id="app" class="demo"></div>

<script>
    const app = Vue.createApp({
        data() {
            return { count: 4 }
        }
    })

    const vm = app.mount('#app')

    document.write(vm.$data.count) // => 4
    document.write("<br>")
    document.write(vm.count)       // => 4
    document.write("<br>")
    // 修改 vm.count 的值也会更新 $data.count
    vm.count = 5
    document.write(vm.$data.count) // => 5
    document.write("<br>")
    // 反之亦然
    vm.$data.count = 6
    document.write(vm.count) // => 6
</script>
</body>
</html>